1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a signal degrade detecting method, a signal restoration detecting method, devices for those methods, and a traffic transmission system, which are suitable for precisely discriminating the signal degrade of a transmission line.
2. Description of the Related Art
A packet network has been constructed in a communication system, and in recent years, demand for enhancing reliability in such networks has been increasing. There has been proposed a communication system in which the signal degrade of a traffic triggers a protection in the packet network as in the related art Synchronous Data Hierarchy (SDH) system. For example, in the Resilient Packet Ring (RPR) that is governed by the IEEE 802.17 standard, the signal degrade of the transmission line is defined as one of a number of protection triggers.
In the packet network described above, the packet communication system such as the ethernet which has a high affinity with the packet network has been frequently employed.
However, packet communication systems including the above-described ethernet present some problems.
A problem with such packet systems is that the packet communication system does not have a function of monitoring a bit error rate as is present in the SDH system. Thus, the signal degrade of the transmission line cannot be detected. Therefore, the transmission line that experiences signal degrade cannot be changed to the normal transmission line by the protection when a error attributable to the signal degrade of the transmission line occurs.
Another problem is that, in the packet communication system, the discarded data rate of the actual traffic, that is, how much traffic volume is not normally transmitted, cannot be measured even if the bit error rate is measured, since the traffic volume to be discarded depends on the length of the packets. Accordingly, even when the bit error rate is being measured, the discarded data rate of the traffic cannot be measured. In the packet communication system including ethernet, the signal degrade of the transmission line is detected by a Frame Check Sequence (FCS) error that performs error detection of the packet. Thus, even if only a part of a packet has an error, the entire packet is discarded. Thus, there arises a problem in that a state in which a large amount of traffic is discarded cannot be actually determined by merely measuring the bit error rate. Specifically, in the case where a large number of packets of the traffic are long length packets, and in the case where a large number of packets of the traffic are short length packets, even if the bit error rates are the same, the traffic volume to be discarded in the former case becomes larger. Also, there is the possibility that most of the packets of the traffic are discarded when a large number of the packets of the traffic are long length packets, even if the generated bit error rate is minimal.
Yet another problem with packet communication systems is that even if the traffic discarded data rate that is attributable to an FCS error of the traffic is measured and the occurrence of the signal degrade can be detected, after the protection of various protocols operate, normal traffic other than the control traffic does not flow in the transmission line. However, control traffic typically has a higher priority and therefore the control traffic continues to flow. As a result, most of the normal traffic disappears, and the discarded data rate of the normal traffic becomes essentially 0. In this case, the signal degrade of the transmission line cannot continue to be detected because there is no normal traffic flowing, and accordingly, a restoration from the signal degrade is mistakenly caused to be detected, and the protection is canceled even when the degrade is not really restored.
The cancellation of the protection leads to another problem that once the protection is canceled, the traffic before the occurrence of the protection flows again in the transmission line, the signal degrade of the transmission line is erroneously detected again, and the protection is erroneously generated again, etc.